Every spring I watch the same thing happen. The weather turns warm in late April, catalogs are open on the kitchen table, and the impulse to plant everything immediately becomes very strong. Then a cold front rolls off Lake Huron and drops the temperature to 28°F on May 3rd.
This is Michigan. The calendar says spring before the thermometer agrees. Understanding the average date of the last spring freeze, and the actual probability of freezing temperatures on any given day, is the most useful thing a Michigan gardener can know.
Click any dot on the map to see the exact average last freeze date for that weather station. Bay City and the Saginaw Bay region appear in mid-green, near the May 2 median on the State Climatologist table.
Map by NOAA Climate.gov, based on 1991–2020 U.S. Climate Normals. Green areas reach frost-safety after the solar start of spring, the darker the green, the later the season. Gray squares indicate locations where freezing temperatures remain likely all spring, or where temperatures are warm enough that frost probability never exceeds 50 percent.
NOAA publishes U.S. Climate Normals based on 30 years of weather station data (currently 1991–2020). For each location and each day of the year, the Normals calculate the historical probability that the temperature will drop to or below 32°F.
The "last spring freeze date" shown on NOAA's interactive map is the date past which the probability of freezing temperatures drops below 50 percent. In other words: past that date, it has historically been more likely to stay above freezing than to freeze again for the rest of the season.
Bay City sits at the southern end of Saginaw Bay in Zone 6a. The proximity to Saginaw Bay moderates temperatures somewhat, especially in spring, the lake absorbs cold air and delays warming, but also buffers against extreme late freezes compared to inland areas at the same latitude.
| Chance of a later freeze | Bay City date | What it means |
|---|---|---|
| 3 springs in 4 | Apr 25 | Most years still freeze after this |
| Half of springs | May 2 | Coin flip: the median last freeze |
| 1 spring in 4 | May 9 | Most years are past freezing |
| Latest on record | May 26 | No spring at the station has frozen later |
Bay City station, 32°F, from the Michigan State Climatologist's freeze probabilities as published by MSU Extension. The same table covers 78 Michigan stations. A 10 percent date is not in that table, so it is not given here.
These dates apply to the 32°F (light freeze) threshold. Tender transplants like tomatoes and peppers can show damage at temperatures as high as 40°F if exposed for extended periods. In practice, I wait until nighttime lows are consistently above 50°F before putting tomatoes in the ground.
Michigan's last spring freeze date varies considerably from south to north and from lakeshore to inland. The Great Lakes exert a moderating effect on western Michigan in particular, pulling warmer air off Lake Michigan and extending the growing season for fruit crops. The Traverse City area, despite its northern latitude, is famous for this, cherries and wine grapes thrive there because of lake-effect warming.
| Station | Half of springs freeze after | 1 spring in 4 freezes after | Latest freeze on record |
|---|---|---|---|
| Detroit (city) | Apr 24 | Apr 30 | May 12 |
| Detroit Metro | May 3 | May 11 | May 29 |
| Ann Arbor | Apr 29 | May 6 | May 27 |
| Grand Rapids | May 2 | May 10 | May 27 |
| Bay City | May 2 | May 9 | May 26 |
| Flint | May 8 | May 15 | May 27 |
| Lansing | May 8 | May 17 | Jun 12 |
| Traverse City | May 23 | Jun 1 | Jul 6 |
| Alpena | May 8 | May 15 | May 29 |
| Marquette | May 13 | May 19 | May 29 |
| Sault Ste. Marie | May 22 | May 29 | Jun 21 |
Michigan State Climatologist freeze probabilities (32°F) via MSU Extension, the same figures as the Michigan frost dates table and the What to Plant Now tool. Corrected October 24, 2026: an earlier version of this table listed dates from an unstated source that disagreed with it, including April 28 for Traverse City.
My raised-bed garden in Bay City runs on a tight Zone 6a schedule. Starting seeds indoors 6–8 weeks before the last frost, I'm typically sowing tomatoes and peppers under lights in late March. The transplants go from 72°F seedling trays to the outdoor beds in stages, a week of hardening off in a protected spot, then into the ground.
My target for outdoor transplanting of frost-sensitive crops is May 15–20, past the 10% probability date, and far enough past the average last freeze that I'm comfortable without row cover. Hardy crops like lettuce, kale, chard, and snap peas go out in mid-April, well before the 50% date.
NOAA's National Centers for Environmental Information publishes freeze probability tables for thousands of U.S. weather stations. To find yours:
You can also use NOAA's interactive map to see the 50% threshold date visualized across the entire United States.
Zone 6a Planting Calendar, full month-by-month schedule for Michigan · Michigan Frost Dates by City, table of 40+ cities · Heirloom Seed Saving Guide, open-pollinated varieties · Seed Starting Guide, timing and technique